1. Field of the Invention
The present invention relates to a display device, in particular, to a structure of an electrode of an active matrix display device that has a light-emitting element.
2. Description of the Related Art
In display devices such as an EL (Electro-Luminescence) display device and a liquid crystal display device, a transparent conductive film such as an indium tin oxide (ITO) film is used as an electrode of a pixel portion.
As a method for processing an ITO film into a desired shape of an electrode, a method of performing wet etching with a resist film as a mask is mainly used.
In general, concerning wet etching of an ITO film, it is known that an amorphous ITO film is subjected to wet etching more easily than a crystalline ITO film, and a structure of an electrode of a liquid crystal display device has been proposed in consideration of the etching characteristic (refer to Patent Document 1, for example).
[Patent Document 1] Japanese Patent Gazette No. 3257913 (page 2 and FIG. 1)
The structure of the electrode of the liquid crystal display device has an upper layer of a crystalline ITO film and a lower film of an amorphous ITO film for improving an etching characteristic.
However, a slight amount of crystalline component during deposition is mixed, for example, in an amorphous ITO film formed with sputtering, and there is a problem of the crystalline component remaining as a residue after wet etching. This tendency is noticeable particularly in the case of using a weak acid solution such as oxalic acid as an etching solution.
However, a low acid-resistant conductive film such as aluminum is used to form a wiring in a display device such as an EL display device or a liquid crystal display device. Therefore, it is preferable to use a weak acid solution for etching an ITO film in order to prevent the conductive film from reacting with an etching solution during etching the ITO film. In addition, also in terms of easiness of using a weak acid solution, it is preferable to use a weak acid solution than a strong acid solution. Consequently, it is required to develop a structure of an electrode or an etching method so that no residue is remaining when a weak acid solution is used for etching.